The present invention generally relates to semiconductor materials and, more particularly, to lattice-matched semiconductor materials for use in electronic and optoelectronic devices.
The interest in photovoltaic (xe2x80x9cPVxe2x80x9d) cells in both terrestrial and non-terrestrial applications continues as concerns over pollution and limited resources continue. Irrespective of the application, and as with any energy generation system, efforts have been ongoing to increase the output and/or increase the efficiency of PV cells. In terms of output, multiple cells or cells having different energy bandgaps have been stacked so that each cell or cell can absorb a different part of the wide energy distribution in the sunlight.
In the prior art, the need to achieve perfectly lattice-matched materials in semiconductor layers of a solar cell or other optoelectronic device is not recognized. Neither is the need to or to control very small amounts of strain in adjacent semiconductor layers. In fact, for example, in prior art examples growing gallium arsenide (xe2x80x9cGaAsxe2x80x9d) and gallium indium phosphide (xe2x80x9cGaInPxe2x80x9d) layers on a germanium (xe2x80x9cGexe2x80x9d) substrate, the lattice constant of the Ge had generally been thought to be sufficiently close to that of GaAs so that GaAs could be grown on a Ge substrate, and GaInP grown on the GaAs, with no detriment to the semiconductor properties.
In fact, the small lattice mismatch between GaAs and Ge causes crystal defects in the GaAs and in the bulk of cells. The presence of such crystal defects reduces the minority-carrier lifetimes in the bulk of the cells, increases the surface recombination velocity at interfaces, and creates possible shunting paths, all of which can reduce the current and voltage of photovoltaic devices, increase the reverse saturation current density and diode ideality factor of p-n junction in the device, and in general, degrade the performance of optoelectronic devices. Further, multi-junction solar cells and other optoelectronic devices having these crystal defects degrade under radiation.
Thus, it is highly desirable to prevent the formation of crystal defects in semiconductor layers and to increase the radiation resistance of multi-junction solar cells and other optoelectronic devices.
It is thus an object of the present invention to prevent the formation of crystal defects in semiconductor layers grown on a growth substrate. It is another object of the present invention to improve or change the characteristics of radiation resistance.
The above objects are accomplished by adding a small amount of an alloying element to a semiconductor material to manipulate the lattice constant of the semiconductor cell to become a perfectly lattice-matched (xe2x80x9cPLMxe2x80x9d) semiconductor that may be used for the base, emitter, back-surface field (xe2x80x9cBSFxe2x80x9d), window, tunnel junction, cap, buffer, and/or other cells in a device. In this context, PLM means that the lattice mismatch between the PLM cell and the growth substrate is less than 0.074%. If specified, PLM may also refer to a difference in lattice mismatch between the PLM cell and an adjacent cell of less than 0.074%.
The PLM semiconductors prevent the formation and propagation of crystal defects in semiconductor devices, or reduce their concentration dramatically. The presence of such crystal defects reduces the minority-carrier lifetimes in the bulk of the cells, increases the surface recombination velocity at interfaces, and creates possible shunting paths. These all can reduce the current and voltage of photovoltaic devices, increase the reverse saturation current density and diode ideality factor of p-n junction in the device, and, in general, degrade the performance of optoelectronic devices. The use of PLM semiconductor cells in the device eliminates or reduces these sources of non-ideal loss, and brings the device performance closer to theoretical limits. The higher degree of lattice matching and the presence of additional elements may be used to improve or change device radiation resistance.
In an alternative preferred embodiment, or in combination with the PLM solar cell layers, the solar cell layers may be grown approximately lattice-matched (xe2x80x9cALMxe2x80x9d) to each other and to the growth substrate. A layer is defined to be ALM if it has an unstrained lattice constant that differs from that of the adjacent subcell layers, or that of the substrate, by an amount greater than or equal to 0.074%, but less than about 0.3%. The ALM layers are thus layers grown with a small intentional mismatch to the substrate or the adjacent subcells.
For some semiconductors, the ability to optimize composition-dependent properties over the wider range of compositions that ALM allows is more advantageous than the lower strain and dislocation density encountered for PLM layers. The small intentional mismatch of ALM cell layers is also expected to result in greater radiation resistance for some semiconductors, so that the balance between high beginning-of-life (xe2x80x9cBOLxe2x80x9d) performance and radiation resistance may be optimized via the degree of small intentional mismatch, for a given solar cell radiation exposure and required service life.